The invention pertains to optical coupling, and in particular, to optical resonant coupling between two waveguides.
There is a need for monolithically integrating various active and passive optical devices to obtain highly functional optical modules. The platform technology to this monolithic integration should be as simple as possible to keep costs low. Currently, the technologies used for monolithic integration, like selective area growth, or regrowth, are not only very expensive but also do not allow for enough freedom in designing the various active and passive devices.
The modes of a laser and an optical fiber are poorly matched in size and shape, leading to poor coupling efficiency therebetween. By integrating a mode expander with a laser, it is possible to obtain efficient coupling to an optical fiber. The invention describes a general technique by which a mode can be coupled from a tightly confined waveguide to a loosely confined waveguide. In particular, the invention employs resonant coupling to achieve mode expansion over a relatively short distance to efficiently couple a tightly confined waveguide in an active region of a semiconductor laser/amplifier and a loosely confined mode in a passive waveguide such as an optical fiber.
Various mode expanders have been demonstrated based on a concept called adiabatic mode transformation. In this approach, there are two separate sections in the device: a section optimized for high gain; and a section optimized for maximum coupling to a fiber. The sections are linked through a mode expander region, which adiabatically transforms, i.e. expands, the mode from the first section to the second section. For minimum losses to occur, adiabatic mode transformation must take place gradually over a relatively long length, e.g. 500 microns. In an exemplary device, as shown in FIGS. 1A and 1B, the adiabatic mode expander is formed as an extension to the active device such as a laser.
Mode transformation can be achieved by means of a technique known as resonant coupling, sometimes hereinafter referred to as phase matching. When two waveguides are approximately matched in their refractive indices and dimensions, and are located in close proximity, there is a transfer of power between the two waveguides in an oscillatory fashion. In other words, the mode in one waveguide is coupled to the proximate waveguide. This phenomenon is illustrated in FIGS. 1C and 1D wherein two waveguides I & II, each 1 .mu.m wide and each with a refractive index of 3.21 are proximately located in side by side relationship. Waveguide I may be an active device having an electrical current input, and waveguide II may be a passive device. The power transfer between the two waveguides is theoretically 100%. If the refractive index of waveguide I is changed, for example, to 3.23 keeping the width unchanged, weak coupling results as the waveguides are no longer phase matched. However, if the width of the waveguide I is changed, for example, to 2 .mu.m, strong coupling is once again observed. This implies that the phase matching condition is re-established. Thus, the phase matching condition depends both on the refractive index and the dimensions of the waveguide. The device, shown in FIGS. 1C and 1D, comprises two rectangular waveguides: active waveguide I and coupling waveguide II. Waveguide I has a higher refractive index than waveguide II, but has a smaller size in both the horizontal (lateral) and vertical (transverse) directions. On the other hand, the waveguide II has a lower refractive index than waveguide I, but has a larger size in both dimensions. The two waveguides can be designed so that their effective refractive indices are nearly equal. This close match of the effective refractive indices forces the power to couple back and forth between the two waveguides in an oscillatory fashion over a characteristic overlapping distance along the length of the waveguides, known as the coupling length L.sub.c.
In principle, by cleaving the device within the coupling length L.sub.c at the exact point where the mode resides in the lower coupling waveguide II, i.e., where transfer takes place, a mode expander can be realized. However, the refractive indices of materials are not known with great precision. Further, the refractive indices also depend on the current injection level in the active device I. Given this uncertainty in the refractive indices, it is not possible to design the waveguides for optimum power transfer. In addition, the point where the mode in waveguide I couples to waveguide II is not known a priori. Therefore, the practical feasibility of the method has not been demonstrated so far. In addition, there is an oscillatory power transfer along the device between the two waveguides due to the phase-matched condition being met along the entire length of the device. Because, there is a power loss during each oscillatory cycle, the device length is usually limited to L.sub.c. This limits the length of the gain (active) region of waveguide I, which in turn, affects the performance of the device.